1. Field of the Invention
The present invention relates to an antenna apparatus for a vehicle, for example, mounted on a vehicle such as a motor vehicle or the like.
2. Description of the Prior Art
As an antenna apparatus provided in the vehicle such as a motor vehicle or the like, a so-called earthed type antenna is conventionally general In this earthed type antenna, a portion on and after a connection portion between a conductor of a coaxial line for the antenna and an earth wire, in other words, a portion from a branch point against the earth wire to a leading end of an antenna element, forms an actual receiver. That is, a receivable portion is provided in addition to the antenna element.
In this case, as a subject to be received by the antenna apparatus for the vehicle, there can be listed up at least an AM radio broadcast wave, an FM radio broadcast wave, a TV broadcast wave and the like. Frequencies of these radio waves are greatly different such that a frequency of the AM radio broadcast wave is about 1 MHz, a frequency of the FM radio broadcast wave is about 76 to 90 MHz, and a frequency of the TV (television) broadcast wave is about 90 to 108 MHz at least in a low band and about 170 to 225 MHz in a high band. Accordingly, wavelengths thereof are greatly different such that a wavelength of the AM radio broadcast wave is about 300 m, a wavelength of the FM radio broadcast wave is about 3 m, and a wavelength of the TV broadcast wave is about 3 m to 60 cm, respectively.
Since it is considered to be ideal that a length of the antenna is a quarter of a wavelength (λ) of the received radio wave, an ideal antenna length is about 15 cm in the case of the TV broadcast wave (UHF wave) having the shortest wavelength among the waves mentioned above. That is, an ideal antenna length is greatly different in correspondence to the subject to be received.
By the way, in the case of the earthed type antenna, since the receivable portion is provided in addition to the antenna element as mentioned above, the other receivable portion than the antenna element becomes long in the case that a distance from the antenna element to an earthed vehicle body is long, and an antenna receiving characteristic is greatly affected at a time of receiving a radio wave having a short wavelength. For example, the ideal antenna length for receiving the UHF is about 15 cm as mentioned above, however, an actual antenna length at a time of being mounted on the vehicle greatly deflects from this value. Accordingly, it is hard to receive the radio wave having a short wavelength such as the UHF or the like, and in the worst case, there is a risk that it can not be received.
For example, in the case of a glass antenna provided in a rear window, only a distance from the earthed portion on the part of the vehicle body to a window glass is frequently more than 15 cm. Therefore, taking the receipt of the UHF into consideration, only this distance is more than the ideal antenna length. Accordingly, in the conventional art, with respect to receiving the radio wave having a high frequency, it is possible to correspond to the receipt by tuning in correspondence to a vehicle type, however, in order to obtain a good antenna characteristic, there is a problem that a lot of trouble is taken for tuning.
In this case, a pole antenna of the vehicle is of an earthed type, however, in this case, since a distance to the earthed vehicle body is very short, an influence of the other receivable portion than the antenna element is restricted minimum.
In relation to the problem mentioned above, it can be considered that the antenna for the vehicle is set to a non-earthed type. In this case, without being particularly interested in the influence of the other receivable portion than the antenna element at a time of receiving the radio wave having a high frequency, for example, there is disclosed in Japanese Patent Laid-Open Publication No. 2001-326515 a structure in which the non-earthed type glass antenna is applied to the glass antenna provided in the rear window.
However, in this conventional structure, since the antenna element is provided dose to the earthed conductor on the part of the vehicle body such as a vehicle body member and a heating filament, there is a hard point that the antenna element is easily affected by a noise.
By the way, in recent years, with regard to an opening and closing body such as a back door and the like and a kind of vehicle body member such as a rear spoiler, a bumper and the like, a structure in which at least an outside plank (a door outer panel or a bumper face) is made of a synthetic resin is put to practical use, in response to an increased request for reducing a weight of the vehicle body.
As mentioned above, in the case that at least the outside plank is made of the resin corresponding to the electrically non-conductive material, with regard to the vehicle body member such as the back door, the rear spoiler, the bumper or the like, it can be considered that an antenna apparatus is provided in an inner portion of the vehicle body member by utilizing the member.
For example, in Japanese Patent Laid-Open Publication No. H10-242733, there is disclosed a structure in which an antenna apparatus is mounted to an inner portion of a rear spoiler made of a synthetic resin.
However, in the case that the antenna apparatus is provided in the inner portion of the vehicle body member mentioned above, it is generally hard to secure a proper antenna length, in connection to a limited space for arrangement.
Then, as one of countermeasure for such a problem, there can be considered that an “L-type antenna” is formed by bending a leading end side of a so-called monopole antenna approximately at right angles.
FIG. 12 shows an example of such an L-type antenna apparatus 80. In this L-type antenna 80, the antenna of the L-type is constituted by a first antenna element E81 extending in a width direction (an up and down direction in FIG. 12) of an antenna substrate 89 from a feeding point Sj of a feeder Fj, and a second antenna element E82 formed so as to be bent at right angles from an end portion of the first antenna element E81.
In this case, in the example shown in FIG. 12, a length of the first antenna element E81 is set, for example, to 100 mm, and a length of the second antenna element E82 is set, for example, to 500 mm, respectively, and an entire length of the antenna is 600 mm.
However, in the antenna apparatus 80 shown in FIG. 12, there is a problem that a good receiving property is exhibited with respect to the FM (radio) broadcast wave, however, a sensitivity comes short with respect to the AM (radio) broadcast wave because the length of the antenna element is short.
Therefore, as shown in FIG. 13, it can be considered that a receiving sensitivity with respect to the AM broadcast wave is improved by extending the second antenna element while maintaining the length (100 mm) of the first antenna element E91 (the extended length of the second antenna element E 92 is 740 mm).
In this case, the entire length of the antenna is 840 mm, so that it is possible to improve the receiving sensitivity with respect to the AM broadcast wave, however, a necessary space in a longitudinal direction of the antenna substrate 99 is increased. Further, there is generated a problem that the receiving sensitivity with respect to the FM broadcast wave is reduced due to an impedance mismatch. In this case, as shown in FIG. 13 mentioned above, the antenna obtained by extending the second antenna element on the basis of the L-type antenna 80 shown in FIG. 12 is called as an “L-type extended antenna” 90.
In connection to this problem, for example, it can be considered to improve the receiving sensitivity with respect to the AM broadcast wave while maintaining the receiving sensitivity with respect to the FM broadcast wave, by adding a coil fox passing the AM frequency band and blocking the FM frequency band to a leading end of the second antenna element E82 in FIG. 12 and adding an antenna element for extension to a leading end of the coil.
However, in this case, there are problems that an antenna structure becomes complex and a manufacturing cost becomes high.
The problem about compatibility in the receiving sensitivity as mentioned above is not limited to the case between the FM broadcast wave and the AM broadcast wave, but exists unavoidably in the case that one antenna receives a plurality of radio waves having different frequencies, for example, the case between the low band and the high band (VHF/High and Low) of the TV (television) broadcast wave or the like.